Sedimentary basins are prone to capture and amplify seismic energy. An important issue of concern for seismic hazard assessment is the amplification effect of the basin on the strong ground motion of earthquakes (Olsen, 2000). The knowledge of the 3D S wave velocity of the shallow structure, especially the quaternary sediments at 0-1 km, is required to understand the seismic response of sedimentary basins (Lai et al., 2020) as it is a main factor affecting the amplification. Meanwhile, unprecedented economic prosperity brought up the rapid development of large cities. As a result, one or more subcenters around the central city are constructed to meet the increasing population growth which cannot be carried by current city areas. High-resolution 3D shallow S wave velocity structure under the ground could provide a guideline for urban planning on earthquake prevention, it is therefore a key element in detailed geological survey before construction and is essential for disaster mitigation in many cities around the world (

In traditional surface wave tomography based on seismic noise, 2D phase or group velocity distribution is obtained by performing pure-path inversion after extracting interstation velocities based on the noise cross-correlation function. In this paper, we show that 2D surface wave phase velocity maps of adequate quality can be obtained directly, without interferometry, by beamforming the ambient noise recorded at array of stations. This method does not require a good azimuthal distribution of the noise sources. The 2D surface wave phase velocity map is obtained by moving the subarrays within a larger dense network of stations. The method is illustrated with seismic noise recorded by over 600 stations of the ChinArray (Phase II). We obtain 2D Rayleigh wave phase velocity maps between 7 and 35 s in Northeastern (NE) Tibetan Plateau and adjacent regions that compare well with results obtained with other methods. The shear wave velocity model is then derived by inverting the phase velocity with depth. The model correlates well with geology and tectonics in NE Tibet. Two clear mid-to-low crustal low-velocity zones are observed at 15-to 35-km depth beneath the Songpan-Ganzi terrane and Northwestern Qilian Orogen, possibly facilitating lower crustal flow in this key region for the tectonic evolution of NE Tibet.

S U M M A R YThe dispersion and the surface displacement as a function of frequency of multiple modes guided waves in stratified media including a low-velocity layer are studied by numerical simulation and experiment. A method is developed to determine the thickness and the shear wave velocity of individual layers. First, the modal analysis of Rayleigh wave is investigated numerically for three layered media. Then, ultrasonic surface measurements are performed for three specimens: Steel half-space, Lucite/Steel half-space and Aluminum/Lucite/Steel half-space. The Characteristics of the dispersion curves are analyzed using the frequency-wavenumber method. The non-dispersive Rayleigh wave is obtained for the first simple specimen. The dispersion curves for two modes are obtained for the second specimen with a low-velocity layer on a fast substrate. The dispersion curves for the third specimen containing a low-velocity layer are apparently discontinuous and correspond to different mode branches. Further analysis demonstrates that the apparent discontinuity is caused by a rapid change of mode excitation with frequency at the surface. While one mode vanishes from the recorded wavefield, the other appears. This indicates that the surface displacements of the modes should be also accounted for in the inverse problem, especially in stratified media with a low-velocity layer. Finally, shear wave velocity profiles are inverted based on the experimental (maybe discontinuous) dispersion curves of fundamental or/and higher modes using a Genetic Algorithm(GA). Besides the dispersion characteristics of each mode, the surface displacement distribution is also taken into account for the case of a low-velocity layer, and as a result, the mode-misidentification is avoided.

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The guided waves in multilayered elastic media are studied. The mechanism of zigzag dispersion curves in the Rayleigh Wave Exploration is analyzed. It is proved that the zigzag dispersion curve can not be obtained by a single guided mode. The relation of the excitation intensity of each mode to the formation parameters is studied. We obtained many results about zigzag dispersion curves that were not available in previous literatures. The effects of the position and thickness of the low-velocity layer and other parameters on the zigzag dispersion curves of Rayleigh wave are also analyzed. Finally, comparative analyses between the practical data and theoretical results are carried out. Theoretical results are consistent to the practical data.